Conventionally, as a reference clock source for an electronic device, such as a microcomputer or cellular phone, a crystal oscillator, such as a temperature compensated crystal oscillator (TCXO), has been used which does not depend on an ambient temperature and/or intrinsic properties of electrical elements and which is superior as a stable oscillation circuit. As load circuits to be connected to the crystal oscillator, many types of load circuits whose load capacities range widely from a small capacity to a large capacity are available. These load circuits are different depending on the application of a user who uses the crystal oscillator.
FIG. 18 is one example of a typical crystal oscillator that has been conventionally used. In this crystal oscillator, the waveform of an oscillation signal generated in an inverter-type oscillation circuit 4, which uses a crystal vibrator X as the oscillation source, is adjusted by an output driving circuit 100, and a signal having a desired output waveform is output from an output terminal OUT. FIG. 19 shows output waveforms of signals output from the output terminal OUT of the output driving circuit in FIG. 18. FIG. 19(a) shows a signal having a pseudo-sine wave, which is called a “clipped sine” for use in cellular phones, and FIG. 19(b) shows a rectangular-wave signal for use in typical digital circuits. FIG. 20 is one example of an output driving circuit having a configuration in which output driving circuits 100, 101, . . . , 10n in the oscillation circuit are connected in parallel at a plurality of stages.
In FIG. 20, the specification of a user-designated output waveform of a signal is determined by one or the combinations of the plurality of the output driving circuits 100, 101, . . . , 10n. Desired characteristics, such as the amplitude of an output waveform, and rising time and falling time characteristics, as specifications for output waveform, are selected by a user, and a signal having a desired output waveform is output from an output terminal OUT. For example, as shown in FIG. 20, disconnecting any of the wiring patterns of the corresponding output driving circuits 100, 101, . . . , 10n, as indicated by the “x” mark in the figure, allows for a change in the waveform characteristics of the output waveform, thereby causing a signal having a desired output waveform to be output from the output terminal OUT.
While the description in the above example has been given for a case in which a crystal oscillator is used, the present invention is not limited thereto. Thus, in conventional, typical oscillators, during manufacture, the wiring pattern of a corresponding output driving circuit is disconnected in accordance with user's specifications to provide a desired output waveform. Thus, to provide a number of output waveforms, the types of output driving circuits are increased, which leads to an increase in manufacturing cost, involving generation of an enormous amount of developing cost or having to control the inventory of many types of output driving circuits. With the conventional crystal oscillator, to meet the requirements of many users, the output driving circuit is designed to be able to deal with a maximum load, and thus the current consumption is inevitably increased. Meanwhile, when the circuit is designed to have reduced current consumption, and connects a load larger than a pre-set load, the circuit provides smaller amplitude of an output signal from the output terminal OUT, and thus it causes a problem in that specifications requested by a user cannot be satisfied.
In general, the duty ratio of the output waveform, which is typically designed and manufactured to be 50%, varies depending on manufacturing variations in transistors included in the output driving circuit and/or a load at a subsequent stage connected to the oscillator. Since the conventional oscillator cannot adjust such variations in duty ratio, it supplies an output signal having an unbalanced duty ratio.
In addition, a conventional cellular phone uses a plurality of oscillators to supply clock signals having frequencies and waveform characteristics which correspond to functional blocks, such as an analog section and a digital section. This results in an enlarged configuration of the entire oscillator and increased power consumption, thus making it impossible to deal with miniaturization and power saving which are in demand these days.